A variety of programmable read only memories are known. Some contain a fusible link which is blown by a high current pulse or irradiation by ultra violet light. Other types contain floating gates wherein a charge is placed on the gate by applying a high voltage, say about 25 volts. Another method is to apply a high reverse voltage to a Zener diode, much higher than its normal breakdown voltage in order to cause the diode to go open circuit.
A problem with all the above types of programmable memory is that they are not suitable for automotive applications wherein in situ programming of the elements is required using only the 12 volt battery power source; furthermore fusible link type memories are not always reliable.
It has been proposed to provide a pn diode junction with a metallic layer nearby whereby the diode in its normal state represents one logic state and the other logic state is created by subjecting the diode to a low voltage current pulse in a forward bias direction which is sufficient to fuse the junction by causing migration of metal ions or atoms into the diode junction region.
A problem with such an arrangement is that said current pulse may destroy the electrical continuity of said metallic layer, resulting in an open circuit. In addition, further metal migration during the lifetime of an otherwise correctly programmed diode junction may destroy electrical continuity.